Mucosal epithelial cells of the small intestine have been identified as a major site for the metabolism of circulating glutamine. The glutamine nitrogen is returned to the circulation as ammonia, alanine, proline, and citrulline. The first three products are utilized by the liver and, in fasted rats, provide about 30% of the nitrogen for hepatic urea biosynthesis. The intestinal citrulline is utilized by the kidneys for the biosynthesis of arginine, 80% of which is released back into the circulation as the major source of endogenous arginine for protein synthesis in all tissues. Liver releases neither citrulline nor arginine under physiological conditions. Small intestine has also been identified as a major source of two of the plasma apolipoproteins, apolipoproteins A-I and A-IV. Production of these proteins by intestine is not dependent upon intestinal fat transport. During fat transport, these proteins reach the blood mainly via intestinal lymph in association with chylomicrons. In the absence of fat transport, however, a larger proportion of these proteins is released directly into the blood, bypassing the lymph.